1. Field of the Invention
The present invention relates to a microwave sensor, more particularly, to a microwave sensor utilized for detecting a range and a velocity.
2. Description of the Related Art
Please refer to FIG. 1, FIG. 1 is a schematic diagram showing a structure of a microwave sensor according to the prior art. The microwave sensor 1 comprises a radio frequency module 10, a demodulation circuit 11, an intermediate module 20, a digital signal processor 30, a transmitting antenna 15 and a receiving antenna 16. The radio frequency module 10 comprises a voltage controlled oscillator 12, a power divider 13, a driving amplifier 14, a low noise amplifier 17, and a mixer 18. The demodulation circuit 11 is used for generating the required demodulation signal, and outputting the demodulation signal to the voltage controlled oscillator 12. The voltage controlled oscillator 12 is an electronic oscillating circuit which controls the oscillation frequency in accordance with the input voltage and finally outputs the frequency modulated continuous wave. The transmitting antenna 15 then emits a transmitted wave. However, before the frequency modulated continuous wave passing the emitting antenna 15, the power divider 13 will input a portion of the power output from the voltage controlled oscillator 12 to the mixer 18 so that an intermediate frequency signal is obtained afterwards. The driving amplifier 14 will amplify the frequency modulated continuous wave before the transmitting antenna 15 receives the frequency modulated continuous wave.
The receiving antenna 16 will input the received signal to the mixer 18 after receiving the echo frequency modulated continuous wave. Before the signal is transmitted to the mixer 18, the low noise amplifier 17 will not only amplify the signal but also inhibit noise as possible to allow subsequent electronic components to perform signal processing. The mixer 18 then calculates the frequency difference between the transmitted wave and the echo wave, down converts the frequency difference and outputs the intermediate frequency signal. After that, the back-end intermediate frequency module 20 regulates the range of detection to obtain the information of the target. Finally, the analog signal is converted to the digital signal and sent to the digital signal processor 30. The digital signal processor 30 thus performs Fourier transforms so as to calculate the target range. If the relative velocity is to be measured, mostly the velocity is derived from the time required for the target to pass or calculated from the frequency difference between frequencies of the transmitting and received beat signals.
However, the emitting antenna 15, the receiving antenna 16, and the radio frequency module 10 in the microwave sensor 1 are first independently designed then integrated together. But the microwave sensor 1 performs detecting based on standard radar principle. In other words, the power level is a major reference for the detection range. At the same time, the size of the antenna directly correlates with the operating frequency. When the operating frequency is lower, the antenna size is larger but the 3 dB beamwidth is larger to result in a wide detection range. Conversely, when the operating frequency is higher, the antenna size is smaller but the 3 dB beamwidth is smaller to result in a narrow detection range. Because the adoption of dual antenna would increase the volume, it is very inappropriate to utilize the dual-antenna architecture in the typically miniaturized sensors. But if the carrier frequency is increased, the 3 dB beamwidth of the antenna becomes smaller, which is inappropriate for the detection of moving targets in a wide-angle environment. In addition, the digital signal processor 30 must perform a large number of Fourier transforms to calculate target range and then the calculation of velocity. Hence, the design of the digital signal processor 30 is more complex and does not fulfill the requirements of the low-cost, small-sized (diameter is smaller than 2 cm), and low power consumption (power consumption is less than 1.5 W) products. Moreover, it must be considered how to measure the velocity under the circumstances of range gates (three-stage range gates).
It is therefore very important to design a new microwave sensor. Not only are the antenna and the ratio frequency module integrated but also the design complexity of the digital signal processor is reduced to satisfy the demand for miniaturization.